Setting the amplification of a secondary-electron multiplier (SEM) in a mass spectrometer generally presents major difficulties. Most spectrometers can measure neither the quantity of ions generated in the ion source nor the amplification of the SEM on their own, because the two can compensate each other over a wide range. If the signal is too large, it is therefore scarcely possible to determine whether too many ions are being generated or whether the amplification of the SEM is set too high. A high SEM amplification is detrimental, however: on the one hand, it reduces the life of the SEM and, on the other hand, the mass spectrum becomes unnecessarily noisy because too few ions are measured. The problem arises because the amplification of a secondary-electron multiplier does not remain constant over its lifetime but is always changing when in use as a result of aging processes. These changes can be continuous, but can also occur in steps of various sizes.
The nature of the problem is explained here using the examples of two completely different mass spectrometers: RF ion trap mass spectrometers as invented by Wolfgang Paul, which mostly use channeltron detectors, and MALDI time-of-flight mass spectrometers, which mainly use multichannel plates as secondary-electron multipliers. The type of SEM is not relevant here. The problem lies in the fact that the rate of ion generation in the ion source or the filling of ion traps and the amplification of the SEM compensate each other in such a way that the SEM amplification cannot be determined on its own. The mass spectrometers do not usually have any other types of measurement devices for ion currents with which the amplification of the secondary-electron multiplier can be determined.
In a MALDI time-of-flight mass spectrometer the ions are generated from solid samples with ionization by matrix-assisted laser desorption. The samples are dried onto a sample support plate and include a mixture of matrix material, usually an easily vaporized organic acid, with very few analyte molecules, which are to be investigated. Bombarding the samples with laser light pulses of a suitable wavelength and suitable pulse duration leads to the generation of a small plasma cloud, in which sufficient ions of the analyte substance are formed in addition to many ions of the matrix substance.
The number of ions generated per laser shot can be varied over wide ranges of several orders of magnitude by changing the laser energy, but only laser energies in a narrow range produce sufficient analyte ions which are relatively stable, i.e., not rapidly decomposing. Analyses with reliable results can only be carried out in this narrow range. The optimum laser energy, on the other hand, depends on the type of matrix material. The laser energy is usually adjusted by measuring the ion current, which includes mainly matrix ions. However, this ion current measurement depends on the multiplier amplification. If the multiplier operation would be always constant and the multiplier would show no signs of aging, its amplification could be set just once at the factory and this would allow the optimum laser energy to always be set during the whole life time of the multiplier. But the secondary-electron multipliers do age, and this is a problem.
A similar problem occurs with ion trap mass spectrometers. In this case it is not the quantity of ions generated but the process of filling the ion trap with ions that is controlled via the ion current at the SEM detector. This filling is critical because even a slight overfill reduces the quality of the mass spectrum, especially the quality of its mass resolving power. The overfill does not simply depend on the number of ions in the ion trap, but is also dependent on the distribution of the ions across the different masses. The filling is therefore controlled by analyzing the preceding mass spectrum, where the numbers of ions for the individual ionic species should be known as accurately as possible. The numbers of ions are again determined using the ion current at the SEM detector. Here, also, interference is caused by the aging of the secondary-electron multiplier because as the amplification decreases, these numbers of ions cannot be determined accurately without resetting the amplification of the SEM.
There are several types of secondary-electron multiplier (SEM, often called “multiplier” for short). In the oldest type, which is still in use, the secondary-electron multiplier includes discrete dynodes, between which voltages in the order of 100 to 200 volts per pair of dynodes are applied by a voltage divider. Secondary-electron multipliers exist with between 8 and 18 dynodes. The ions impinge on the first dynode, thus generating secondary electrons, which are accelerated and then impinge onto the second dynode. Each of these electrons then generates, on average, several secondary electrons so that an avalanche of electrons forms along the dynodes. The amplification is the number of electrons from the last dynode per ion which impinges onto the first dynode. The amplification of commercially available multipliers can be adjusted over a wide range, in the extreme case between 104 and 108, by changing the total voltage, although operating the multiplier at the highest voltages generally leads to very rapid aging.
Other types of secondary-electron multipliers are the so-called “channeltron multipliers” and the “multichannel plates”. The channeltron multiplier includes of a single channel with an opening in the form of a trumpet, the channel bent to a kind of spiral. The multichannel plate is usually supplied in a design that includes two plates, each including millions of parallel channels, one behind the other with channel directions at a slight angle to each other (chevron arrangement). In both these types of secondary-electron multiplier, voltage drops exist across the internal surface of the channels which, given an appropriate shape and surface conditioning, lead to electron avalanches in the channels. The amplification ranges are similar to those of dynode secondary-electron multipliers. FIG. 1 shows the characteristic for a double multichannel plate with channels only two micrometers in diameter.
The secondary-electron multipliers have characteristics displaying the logarithm of the amplification as a function of the supply voltage. The characteristics are more or less straight, i.e., an increase in the supply voltage by a value ΔV increases the amplification by a factor F. Aging changes the position of the characteristic, but its gradient stays approximately the same. A decrease in the amplification by a factor F as a result of aging can therefore be compensated again to a certain extent by increasing the voltage by a voltage difference ΔV.
It is an unfortunate fact that the amplification of all secondary-electron multipliers deteriorates during their service life. This aging does not simply depend on time, but on the duration of use, the type and energy of the ions which generate the first generation of electrons, and further parameters such as temperature, resting periods, type of residual gas in the vacuum, venting periods, et cetera. Their amplification, which depends on the voltage applied, must therefore be occasionally readjusted by adjusting this voltage. There is a need for an automated adjustment procedure that is run regularly in the mass spectrometer.